Influence of electron confinement effects on the band gap of almost monatomic EuS$_2$ layers

Europium disulfide is a layered semiconductor with a quasi-ionic bond type. Previously, it has been demonstrated that almost monatomic films can be formed from this material by mechanical splitting. In this work, the most energetically favorable structure of monatomic films is established using ab initio calculations, and the behavior of the band gap depending on the number of monolayers in the film is studied. To establish the role of nonlocal corrections and corrections associated with the spin–orbit interaction, the calculation results are compared with the position of the direct fundamental absorption edge of bulk crystals estimated from the experimental hot photoluminescence and microreflection spectra. It is found that the indirect character of the band gap is also retained in thin films. The confinement effects (dimensional localization of electrons) cause inhomogeneous broadening of the band gap over the Brillouin zone. The gap width almost does not change between the bulk material and its films at the edges of the Brillouin zone, and a significant change occurs only in the center of the Brillouin zone. A singularity in the density of states caused by the equalization of the energies for the D and E$_0$ points of the Brillouin zone is predicted in EuS$_2$ films about 10 ML thick.